Semiconductor wafers and other items oftentimes require extremely high levels of cleanliness. Specifically, during the manufacture of semiconductor circuits, microscopically small particles remain on the surface of the wafer structure. Sometimes these particles are called “fallen defects”, and, if not removed, can cause the circuit to operate incorrectly, or not at all. Therefore, as many of these fallen defects as possible should be removed from the semiconductor surface.
One method for cleaning fallen defects from wafers is to place wafers near a rod powered by megasonic energy, and to move the wafer underneath the rod. The megasonic energy promotes the rod to create a cleaning action, which lifts the fallen defects from the wafer surface. Megasonic energy is energy having a frequency about 10-50 times as high as Ultrasonic energy, e.g., in a range from about 200-1000 kHz. At this frequency, good cleaning properties are achieved at a power level of between about 0.01-0.10 W/mm2 of the wafer surface. Additionally, chemicals such as water or other solutions can be applied to the surface of the wafer so that the solution is between the wafer and the cleaning rod during the time the cleaning rod is powered. Further, the wafer can be rotated below the powered cleaning rod to promote the cleaning action. An example of a wafer cleaning system by megasonic energy is described in U.S. Pat. No. 6,039,059 to Bran and its progeny, which are hereby incorporated by reference in their entirety.
FIG. 1 is a cross-sectional view showing an example of a wafer 10 set to be cleaned by a cleaning rod, or probe 20. A cleaning solution 12 is present on the surface of the wafer 10, and creates a meniscus in areas around the cleaning probe 20. The meniscus may be nonsymmetrical as the wafer 10 spins below the cleaning probe 20.
The cleaning probe 20 plays a key role in transmitting the high frequency energy into the cleaning solution 12 that is located between the probe and the wafer 10. When megasonic energy is applied to the cleaning probe 20, a cavitation effect is produced in the cleaning solution 12 whereby bubbles form and grow in the solution during one-half cycle of a wave and collapse in the other one-half cycle. Particles are lifted from the surface of the wafer 10 as the bubbles are produced and collapse, and are carried away by the cleaning solution 12, thereby cleaning the surface of the wafer.
One problem that occurs during the cleaning by high frequency energy is that the cleaning action can damage the wafer 10, or structures produced on the wafer, termed pattern damage. More wafer pattern damage is observed at locations directly beneath the cleaning probe 20 than in areas not beneath the probe. It is thought that damage is caused by the megasonic waves projecting directly beneath, or normal incident to the cleaning probe 20 rather than the megasonic waves projecting more transversely to the probe. When the cleaning probe 20 transmits megasonic energy into the cleaning solution 12, some energy is perpendicularly reflected from the wafer 10 surface back toward the cleaning probe 20. This reflected energy probably causes constructive interference with the additional (continuous) energy supplied by the cleaning probe 20, and/or oscillates between the cleaning probe and the wafer 10. It is believed that these megasonic interference oscillations cause the damage to the wafer 10.
Some wafer cleaning systems try to minimize this damage and increase the amount of cleaning action by spinning the wafer 10, for example between 15 and 30 revolutions per minute (RPMs), while the probe 20 is powered for megasonic cleaning. Although the amount of damage is lessened by cleaning the wafer 10 as it spins, the damage is not eliminated. Additionally, it has been found that if the wafer is spun too quickly, for example greater than 50 RPM, then the fallen particles are not effectively cleaned from the wafer 10 surface. If the wafer 10 is spun too quickly, for example greater than 50 RPM, the thickness of the cleaning solution 12 becomes so thin that adequate megasonic energy cannot be transmitted from the cleaning probe 20 to the surface of the wafer.
A distance between the cleaning probe 20 and the wafer 10 surface is also shown to have an effect on the cleaning efficiency and the amount of damage caused to the wafer. It has been determined that the most effective distance between the bottom edge surface of the cleaning probe 20 and the wafer 10 surface is approximately three-fourths of the wavelength of the megasonic energy used to excite the cleaning probe. At this distance, the cavitation effect in the cleaning solution seems to be the most efficient. Given a megasonic energy wavelength of about 900 khz, the optimum distance between the cleaning probe 20 and the wafer 10 surface is therefore approximately 1.65 mm. Distances greater than the optimum tend to not clean the wafer 10 surface very well, while distances less than the optimum tend to cause more damage to the wafer.
With these distances in mind, another idea used to more effectively clean the wafer 10 surface is to etch a pattern of transverse grooves 23 along the bottom edge surface of a cleaning probe 22, as illustrated in FIG. 2. The grooves 23 placed in the cleaning probe 22 tend to increase the overall average distance between the cleaning probe 22 and the wafer 10 surface, while maintaining the cleaning action of the areas of the wafer surface that are not directly under the probe 22. That is to say, the grooves 23 in the cleaning probe 22 play a part in reducing megasonic energy transmitted from the bottom surface of the cleaning probe 22 to the wafer 10 surface directly beneath the bottom surface of the cleaning probe, without reducing the megasonic energy transmitted from the cleaning probe 22 to areas of the wafer 10 other than directly below the probe. By having grooves 23 in the cleaning probe 22, the cleaning action is preserved while the damage to the wafer 10 surface is lessened because of the greater average distance between the wafer 10 surface directly beneath the grooved cleaning probe 22 and the cleaning probe 22 itself. However, damage can still occur at the wafer 10 surface, most likely due to the concentration of megasonic energy at the edges of the fixed grooves 23 in the cleaning probe 22.
Embodiments of the invention reduce the amount of damage caused during the cleaning of wafers, while maintaining current standards of cleanliness.